Field of the Invention
The disclosure generally relates to an operation method for RRAM (Resistive Random Access Memory), and more particularly, to an operation method for improving the reading accuracy of the RRAM.
Description of the Related Art
Memories can be divided into two main types: volatile memory and non-volatile memory. Among the non-volatile memory, flash memory is widely utilized. However, flash memory has some disadvantages, which include its high operating voltage, low operating speed, and low reliability. Regarding advanced research into memory, RRAM is one of the most widely-researched subjects. RRAM has many advantages, such as its simple structure, low operating voltage, high operating speed, multi-bit memory, high reliability, small element area, non-damaging reading, and low cost.
Although the programming voltage pulse of the RRAM is about 20 ns during the setting state, and it is about 50 ns during the reset state, the distribution of the value of resistances of the RRAM is very wide. In order to effectively control the value of resistance, a read verification is needed between the programming voltage pulses. Because of the diffusion of oxygen between traps and the effect of tightening and loosing, the reading of RRAM is not stable, which results in reading errors. FIG. 1A is a schematic diagram of the reset current (Ireset) and the reading number. FIG. 1B is a schematic diagram illustrating the first reset current and the second reset current. The predetermined current is assumed to be 1 uA. When the reset current is greater than the predetermined current, this indicates that the verification has failed. Regarding FIG. 1A, the verification of the first bit fails after it is read the second time, and the second bit and the third bit are verified successfully. In addition, FIG. 1B illustrates that the correlation coefficient R of the first reset current and the second reset current is 0.92. Specifically, most of the first reset currents are smaller than 1 uA, but some second reset currents are greater than 1 uA. Therefore, the reset current is not stable and has a wide distribution.
Therefore, an operation method of RRAM is needed to decrease the number of verification failures when reading, and improving the stability of reading.